This invention relates to an electronic measurement system including both wideband AC and DC measurement devices and particularly to a circuit for such a system which maintains both AC and DC measurement accuracy while both measurement devices are simultaneously connected to a single input.
In electronic test and measurement applications, it is often necessary to measure wideband AC characteristics and DC characteristics of a given electrical signal. Even though it is highly desirable to make both wideband AC and DC measurements through a common probe, input, and attenuator system, because of interference between the two types of measurement devices, this has resulted in undesirable compromises.
Wideband instruments, such as oscilloscopes, have excellent AC and transient waveform measurement capability because the input circuit has carefully controlled resistance and capacitance loading. Any additional loading, whether resistive or reactive, will cause an impedance mismatch and undesirable waveform distortion. For this reason, precision DC measuring devices, such as digital multimeter devices, are not connected to the input circuit of wideband oscilloscopes. Existing methods for combined oscilloscope and digital multimeter testing require separate inputs, thus requiring separate test probes. Other methods attempt to monitor DC voltage after the input stage of the oscilloscope to maintain wideband and transient response but at the sacrifice of DC measurement accuracy.
Alternatively, an oscilloscope may be used to measure DC voltage to a limited accuracy by grounding the input through an AC-DC-ground switch to establish the DC reference level. This is used to null drift of the input amplifier when measuring the incoming DC signal.
These methods are cumbersome, yet commonly used.